1. Field of the Invention
The present invention relates to an angle sensor including a magnetoresistive element, such as a GMR element, and particularly to an angle sensor that can reduce the waveform distortion of output voltage.
2. Description of the Related Art
Angle sensors including magnetoresistive elements are already known (see, for example, Japanese Unexamined Patent Application Publication No. 2002-303536).
The magnetoresistive element is a laminate that essentially consists of an antiferromagnetic layer, a pinned magnetic layer, a nonmagnetic material layer, and a free magnetic layer. The pinned magnetic layer is unidirectionally magnetized by a coupling magnetic field generated between the pinned magnetic layer and the antiferromagnetic layer. The magnetization of the free magnetic layer varies under an external magnetic field.
The angle sensor may be provided with a rotor having a magnet over the magnetoresistive element. The rotation of the rotor changes the direction of a magnetic flux flowing into the laminate from the magnet.
The magnetization of the free magnetic layer varies with the direction of the magnetic flux, and thereby the electrical resistance and therefore the output voltage of the magnetoresistive element vary. The angle sensor detects the rotation angle based on the change in the output voltage.
FIG. 7 shows a magnetoresistive element for use in a conventional angle sensor.
Two magnetoresistive elements 2 and 3 are disposed on a substrate 1. Each end of the magnetoresistive elements 2 and 3 is connected to a lead 4, 5, 6, or 7 made of Au or the like. The leads 4, 5, 6, and 7 have terminals 8–15 at their opposite ends.
Four substrates 1, for example, are disposed below the rotor. Any magnetoresistive elements on the substrates 1 are connected to one another to form a Wheatstone bridge.
The magnetoresistive elements 2 and 3 extend in the X-axis direction (the width direction of the substrate 1), and meander in the Y-axis direction (the depth direction of the substrate 1), thus forming a single zigzag structure.
The electrical resistance of the magnetoresistive element is expressed by the following equation:Rg=Rg0−ΔRg cos θ  (1)wherein Rg is an electrical resistance of the magnetoresistive element, Rg0 is a center resistance, ΔRg is an amplitude of the electrical resistance change, and θ is an angular difference in the magnetization direction between the pinned magnetic layer and the free magnetic layer.
To obtain a theoretical waveform of output voltage, the magnetization direction of the pinned magnetic layer should be independent of the external magnetic field, and the magnetization direction of the free magnetic layer should be identical to the direction of the external magnetic field. However, in practice, the magnetization direction of the free magnetic layer is not identical to the direction of the external magnetic field. This, in combination with other factors, causes the measured waveform of output voltage to deviate slightly from the theoretical waveform.
FIG. 8 shows a waveform of the output voltage generated by the rotation of a rotor that has a magnet and faces the substrate 1 in FIG. 7. The measured waveform of output voltage deviates from the theoretical waveform.
Such a distortion of the waveform may partly result from AMR (anisotropic magnetoresistance) effect, which is illustrated in FIG. 9. When an electric current runs through a GMR element aligned in the X-axis direction, and a free magnetic layer is magnetized at an angle of θ′, the AMR effect is expressed by the following equation:RA=RA0+ΔRA cos2θ′  (2)wherein RA is a contributing resistance by the AMR effect, RA0 is a center resistance, ΔRA is an amplitude of the contributing resistance change, and θ′ is an angle between the current direction and the magnetization direction of the free magnetic layer.
As is apparent from equation 2, RA changes with θ′. This AMR effect causes variations in Rg in equation 1, and thus the waveform distortion cannot be reduced.
Another factor in the waveform distortion is a magnetic field having a shape anisotropy. The magnetic shape anisotropy is a property that magnetization tends to be oriented longitudinally, for example, along the X-axis of the magnetoresistive element 2 or 3 in FIG. 7.
FIG. 10 illustrates a deviation in the magnetization direction in the free magnetic layer, caused by the shape anisotropic magnetic field. Considering the shape anisotropic magnetic field Hk, the magnetization direction θ″ of the free magnetic layer is expressed by the following equation:
                              θ          ′′                =                              TAN                          -              1                                ⁢                                                    H                k                            +                                                H                  0                                ⁢                SIN                ⁢                                                                  ⁢                θ                                                                    H                bf                            +                                                H                  0                                ⁢                COS                ⁢                                                                  ⁢                θ                                                                        (        3        )            wherein H0 is an external magnetic field, Hk is a shape anisotropic magnetic field, Hbf is a bias magnetic field, θ is an external magnetic field direction, and θ″ is a magnetization direction of a free magnetic layer.
Ideally, as described above, the magnetization direction of the free magnetic layer is identical to that of the external magnetic field H0. However, as shown in FIG. 10, the shape anisotropic magnetic field Hk, as well as the bias magnetic field Hbf, causes the angle θ″ of the free magnetic layer (the angle θ″ is an inclination from the Y-axis) to deviate from the angle θ of the external magnetic field H0.
Thus, to minimize the deviation, the shape anisotropic magnetic field Hk is preferably as small as possible.